in case the rotation is 0, `v >> (32 - r)` would end up doing a 32 right
shift which is equal to the width of `v` and thus undefined behavior.
ref: https://blog.regehr.org/archives/1063
* icons-hash: take total probe count into account as well
* icons-hash: use a better PRNG
the older method was using a multiplicative congruential generator (MCG)
which doesn't work too well especially with just 32 bits of state.
change it to a PCG instead with 64 bits of state (and 32 bits of output)
which should give better results.
and since we should get better rng - the search iteration has been
halved as well to save some build time.
* icons-hash: use an xor-rotate hash function
* icons-hash: fix some compiler warnings
a lot of the extension use the same icon. this can be exploited via
having an array with all the unique icons and then storing a single byte
index into the unique array.
when using `O_EMOJI` this results in around ~1.7KiB drop in the total
table size. `O_NERD` and `O_ICONS` get roughly ~0.5KiB savings.
This pretty much reworks the entire icon system. Some notable changes:
* The extensions are put into a statically generated hash-table instead
of a sorted array. We use Robin-Hood insertion to reduce the max probe
length. Currently we need to probe only 2 slots for `O_EMOJI` and only
3 for `O_NERD`/`O_ICONS`.
* I've opted not to use a perfect-hash since the perfect hashes
generated by [`gperf`](https://www.gnu.org/software/gperf) used some
huge lookup table. The hash function also wasn't as minimal as I'd
like.
* Colors are now using X-Macros. This should speed up startup since we
don't have to search `icons_ext` linearly to find unique colors.
* The hash-table generator outputs a more space optimized `struct
icon_pair` using a char array instead of char pointer. This brings
down the binary size from `145KiB` when using `O_NERD` down to
`137KiB`.
* Some unnecessary duplication and indirection has been reduced by using
the `ICON_STR()` macro.
